The present invention relates to an improvement for an amplifier, and particularly to an amplifier circuit that operates in both a saturated and a linear mode.
Power amplifiers see usage in many different areas of everyday life. One of the more common applications is in increasingly ubiquitous mobile terminals, such as cellular phones, personal digital assistants, and the like. In such devices, an amplifier chain is usually positioned immediately prior to an antenna and boosts a signal for wireless transmission to a base station.
Careful control must be exercised over the amplification so as to avoid adjacent channel interference. This is done in part through power control signals sent by base stations to the mobile terminals. However, such arrangements leave the actual power control to the mobile terminal, which must boost the power at the desired frequencies while suppressing leakage into other frequencies.
Recent years have seen an increase in the number of mobile terminals. Coupled to this increase was a demand for more bandwidth in which these mobile terminals could operate. To that end, various government entities set aside additional bandwidth in the electromagnetic spectrum. However, this bandwidth is not contiguous to the original cellular bandwidth. Still further complicating the matter are the various digital and analog standards that may be present within the various bandwidths. As yet another problem, not every nation has the same bandwidths set aside for the same standards. While recent efforts at harmonization have seen some improvement, the fact remains that many legacy systems are still existent and require mobile terminals that operate in two or three different bands or modes of operation. Even where the bandwidth is well settled, there are also evolving modulation schemes within a given bandwidth. The newer modulation schemes allow more information to be transmitted over the wireless communication link.
What may be an effective amplification scheme in a first mode of operation may be inefficient or undesirable in a second mode of operation. Duplication of amplifier chains is wasteful not only of components, but also of valuable space within the mobile terminals that are being designed to be smaller with each iteration. Newer modulation schemes may require different amplification schemes than the older modulation schemes.
Thus a need remains for an amplifier circuit that can switch between modes effectively while still providing the desired amplification.
The present invention provides at least three techniques by which a power amplifier chain may be switched between different modes of operation. The power amplifier chain, in an exemplary embodiment, comprises three amplifier stages arranged in series. The first amplifier stage is associated with a switch.
In a first embodiment, when the switch is open, each amplifier stage of the amplifier chain operates in a saturated mode. When the switch is closed, the bias on the remaining two stages is increased, thereby increasing the gain supplied by the remaining two stages. Further, the closed switch creates a short around the first amplifier stage, allowing the remaining two amplifier stages to operate in a linear mode.
In a second embodiment, when the switch is open, each amplifier stage of the amplifier chain operates in a saturated mode. When the switch is closed, a feedback loop is created around the first amplifier stage, thereby reducing the gain of the first amplifier stage. The bias on at least the remaining two stages is increased, and the amplifiers in the chain may operate in a linear mode.
In a third embodiment, the feedback loop of the second embodiment is present and the bias on the stages is varied. Further, the third amplifier stage comprises a plurality of transistors operating in parallel, some of which are dynamically taken in and out of use so as to control the gain of the last amplifier stage.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.